U.S. patent number 4,208,737 [Application Number 05/967,179] was granted by the patent office on 1980-06-17 for low frequency inertia balanced dipole hydrophone.
This patent grant is currently assigned to Westinghouse Electric Corp.. Invention is credited to George R. Douglas, John H. Thompson.
United States Patent |
4,208,737 |
Thompson , et al. |
June 17, 1980 |
Low frequency inertia balanced dipole hydrophone
Abstract
A dipole hydrophone having a differential pressure sensing unit,
for example, a multi-laminar bender disc, within a liquid filled
housing. Two liquid filled acoustic waveguides form extensions of
the housing and include pressure sensing ports. A mass of
predetermined value is connected to the sensing unit and with a
predetermined separation between sensing ports, the mass value is
chosen so that the sensing unit response to acceleration is very
nearly equal and opposite to its response due to the inertial mass
of the liquid.
Inventors: |
Thompson; John H. (Severna
Park, MD), Douglas; George R. (Arnold, MD) |
Assignee: |
Westinghouse Electric Corp.
(Pittsburgh, PA)
|
Family
ID: |
27123943 |
Appl.
No.: |
05/967,179 |
Filed: |
December 6, 1978 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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815387 |
Jul 13, 1977 |
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Current U.S.
Class: |
367/171; 310/337;
367/174 |
Current CPC
Class: |
B06B
1/0603 (20130101); B06B 1/0655 (20130101); G01V
1/38 (20130101) |
Current International
Class: |
B06B
1/06 (20060101); G01V 1/38 (20060101); H04B
013/00 () |
Field of
Search: |
;340/8-14
;310/329,330 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Tudor; Harold J.
Attorney, Agent or Firm: Schron; D.
Parent Case Text
This is a continuation of application Ser. No. 815,387, filed July
13, 1977, now abandoned.
Claims
We claim as our invention:
1. A hydrophone comprising:
(A) a liquid filled housing;
(B) differential pressure sensing means within said housing and
having a certain mass;
(C) liquid filled means in fluid communication with said sensing
means for communicating the pressure at spaced apart locations to
said sensing means; and
(D) said hydrophone being constructed and arranged that, when
accelerated, the inertial response of said liquid on said sensing
means is approximately equal and opposite to the inertial response
of said sensing means due to said mass.
2. A hydrophone comprising:
(A) a liquid filled housing extending along a central axis;
(B) differential pressure sensing means within said housing,
coaxial with said axis and having first and second pressure sensing
sides;
(C) liquid filled means for communicating the pressure at spaced
apart locations to opposite sides of said sensing means; and
(D) said hydrophone being constructed and arranged that, when
accelerated, the resultant inertial force of said liquid acts on
one of said sides in the same direction as the axial component of
said acceleration.
3. Apparatus according to claim 2 which includes:
(A) weight means added to said sensor means and being of such value
as to balance said inertial force of said liquid.
4. A hydrophone comprising:
(A) a housing;
(B) a differential pressure sensing means within said housing
defining left and right chambers;
(C) said sensing means having left and right pressure sensing
sides;
(D) left and right acoustic waveguides coupled to said housing and
having respective pressure sensing ports;
(E) a liquid contained within said waveguides and housing; and
(F) said housing including passageways so as to communicate the
liquid and sensed pressure of said right waveguide to said left
chamber and left pressure sensing side, and to communicate the
liquid and the sensed pressure of said left waveguide to said right
chamber and right pressure sensing side.
5. A hydrophone comprising:
(A) a housing;
(B) a differential pressure sensing means within said housing and
defining first and second separate chambers;
(C) first and second acoustic waveguides coupled to said housing
and having respective pressure sensing ports;
(D) a liquid contained within said waveguides and housing
communicating the pressures at said ports to respective ones of
said separate chambers;
(E) said sensing means having a certain mass that, when said
hydrophone is accelerated said sensing means tends to provide a
first output voltage of a first polarity;
(F) said sensing means tending to provide a second output voltage
of opposite polarity in response to the inertial force of said
liquid, on said sensing means due to said acceleration; and
(G) said mass being of such value that said first and second output
voltages are approximately equal so as to tend to cancel the effect
of said acceleration.
6. Apparatus according to claim 5 wherein:
(A) at least one of said waveguides is adjustable so as to vary the
linear distance between its measuring port and said sensing
means.
7. Apparatus according to claim 6 wherein:
(A) said housing includes a liquid filled aperture to allow for
overflow of said liquid when said distance is decreased and to
allow for liquid addition when said distance is increased.
8. Apparatus according to claim 5 wherein:
(A) said pressure sensing port of a waveguide is at the end of said
waveguide.
9. Apparatus according to claim 5 wherein:
(A) each said waveguide has a closed end; and
(B) said pressure sensing ports are displaced from said ends,
toward said housing.
10. Apparatus according to claim 5 wherein:
(A) said sensing means is a multi-laminar bender disc and which
includes weight added to either side of said disc.
11. Apparatus according to claim 5 wherein:
(A) said sensing means is a piezoelectric cylinder.
12. Apparatus according to claim 5 wherein:
(A) the linear distance from the port in one said waveguide to said
sensing means is equal to the linear distance from the port in the
other said waveguide to said sensing means.
13. Apparatus according to claim 5 wherein:
(A) the linear distance from the port in one said waveguide to said
sensing means is greater than the linear distance from the port in
the other said waveguide to said sensing means.
14. Apparatus according to claim 12 wherein:
(A) the length of one said waveguide is greater than the length of
the other said waveguide.
15. Apparatus according to claim 14 wherein:
(A) said lengths are in the ratio of 3:1.
16. Apparatus according to claim 5 wherein:
(A) said housing and waveguides are symmetrically disposed about a
longitudinal axis.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention in general relates to hydrophones, and in particular
to a dipole hydrophone with a low vibration sensitivity, a high
acoustic sensitivity and a low flow noise response.
2. Description of the Prior Art
Dipole hydrophones find extensive use in the underwater environment
for listening to very low frequency noise as may be produced for
example, by a submarine. The dipole hydrophone is positioned at
some point in the water, either alone or as a part of an array, and
provides an output signal in response to received acoustic signals
in accordance with its beam pattern in the form of a figure
eight.
Most dipole hydrophones respond directly to particle velocity and
any mechanical vibration acceleration from the support structure
may tend to provide an unwanted output signal.
In copending application Ser. No. 352,820, filed Apr. 19, 1973, and
assigned to the same assignee as the present invention, there is
described a dipole hydrophone which utilizes two masses having
different ratios of actual mass to added radiation mass with each
being connected by means of a multi-laminar magnetostrictive arm to
a base member, with the unit including a number of pickups for
providing an output signal. This hydrophone significantly reduces
the effects of acceleration, however, it does require two matched
multi-laminar arms and two matched pickup units.
To eliminate the particle velocity response, a dipole hydrophone
has been proposed which responds to the pressure gradient of an
acoustic wave by means of two monopoles separated by a half
wavelength and connected so that the signals from the monopoles
subtract. Although the arrangement has very desirable inertia
balancing properties, there are disadvantages. For example, the
sensitivity is limited by the thermal noise of the preamplifiers
utilized in the signal processing. A difference signal may be
extremely small compared with this thermal noise. Further, in order
to obtain an accurate output, the monopoles and signal processing
channels must be very accurately balanced.
In a somewhat analogous art, a pressure gradient microphone has
been proposed which includes a housing containing a differential
pressure sensor and includes elongated first and second arms
extending from the housing to spaced apart points where the
respective pressures are communicated to either side of the
differential pressure sensor. Such arrangement, to be described in
FIG. 2, is air or gas filled and has a high acoustic sensitivity
with low response to flow noise. The arrangement, however, is not
suitable for underwater use; however, even if filled with a liquid
and operated underwater, the unit would be extremely sensitive to
vibrations.
SUMMARY OF THE INVENTION
A pressure gradient dipole hydrophone is provided which has a very
low vibration sensitivity, a high acoustic sensitivity, and a low
flow noise response. The hydrophone includes a liquid filled
housing having a differential pressure sensing means within the
housing. Liquid filled acoustic waveguides are coupled to the
housing and include respective pressure sensing ports whereby the
respective pressures at said ports are communicated to respective
sides of the differential pressure sensing means.
The construction of the hydrophone is such that when accelerated,
the inertial response of the liquid on the sensing means is
approximately equal and opposite to the inertial response of the
sensing means due to its mass. In most instances, mass will be
added to the sensing element and the distance between pressure
sensing ports adjusted until little or no voltage is provided by
the sensing means when the hydrophone is vibrated.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the directivity pattern of a dipole
hydrophone;
FIG. 2 is an axial, cross-sectional view of a pressure gradient
microphone of the prior art;
FIG. 2A illustrates the deflection of the differential pressure
sensing element of FIG. 2 as a result of its own mass, in response
to acceleration of the unit in the direction illustrated, and FIG.
2B illustrates its deflection due to liquid inertial force;
FIG. 3 is an axial cross-sectional view, in simplified form, of an
embodiment of the present invention;
FIG. 3A illustrates the differential pressure sensor deflection as
a result of its own mass, in response to acceleration in the
direction illustrated and FIG. 3B illustrates its deflection due to
liquid inertial force;
FIGS. 4A through 4F illustrate liquid filled containers to aid in
an understanding of the pressure considerations herein;
FIG. 5 is an exploded view of one embodiment of a differential
pressure sensing means which may be utilized herein;
FIG. 6 is a plan view, with a portion broken away, of one
embodiment of the present invention;
FIG. 7 is an exploded view of the hydrophone of FIG. 6;
FIG. 8 is a sectional view of the portion of the housing
illustrated in FIGS. 6 and 7;
FIG. 9 illustrates an alternate embodiment of the acoustic
waveguide extension illustrated in FIGS. 6 and 7;
FIGS. 10 and 10A are simplified versions of another embodiment of
the present invention;
FIG. 11 is the beam pattern obtained with the apparatus of FIG.
10;
FIG. 12 is an axial cross-sectional view through an acoustic
waveguide for the embodiment of FIG. 10;
FIG. 13 is an axial cross-sectional view of a simplified version of
another embodiment of the present invention, and FIG. 13A is a view
along line AA of FIG. 13; and
FIGS. 14 and 15 illustrate another type of differential pressure
sensing element in the form of a cylinder.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, the dipole hydrophone, also known as a
doublet transducer, may be represented by two small, closely spaced
transducers indicated by points 10 and 10', having opposite
polarity. The signals from these two points cancel for equal
pressure, thus any net response is due to a pressure gradient
across the dipole. If points 10 and 10' are small with respect to
the operating wavelength, and if the distance d between them is
also small in comparison with the wavelength, for example less than
or equal to .lambda./2, the directivity pattern will be the figure
eight pattern, 12, also known as a cosine directivity pattern
wherein the response is proportional to the cosine of the angle
.theta..
FIG. 2 illustrates a prior art pressure gradient microphone, as
opposed to a hydrophone. The microphone includes a housing 12 with
a differential pressure sensor 14 contained therein separating the
housing into two distinct chambers 16 and 17. The differential
pressure sensor 14 may be in the form of a multi-laminar bender
disc made up of a disc of metal sandwiched between two
piezoelectric discs.
First and second acoustic waveguides 20 and 21 are coupled to
housing 12 and include respective pressure measuring ports 22 and
23 covered by compliant members 24 and 25.
The housing and waveguides are filled with a gas having a high
propagation velocity, hydrogen or helium being examples, and the
pressures at ports 22 and 23 are communicated to respective sides
of the differential pressure sensor 14, which then provides an
electrical output signal indicative of any pressure difference.
The microphone has very high acoustic sensitivity with a very low
response to flow noise. However, the unit could not be operated at
deep ocean depths since the compliant covers would collapse.
Replacing the gas with an electrically insulating liquid results in
a dipole hydrophone which has a low response to flow noise,
good-sensitivity, but is highly sensitive to vibrations. For
example, let it be assumed that the unit is accelerated in the
direction indicated. The sensor deflection from its own mass is
illustrated in FIG. 2A. As a result of the acceleration, the liquid
inertial pressure also operates on the sensor and deflects it as
illustrated in FIG. 2B. This deflection illustrated in FIGS. 2A and
2B will cause an unwanted output signal which is due solely to
movement or vibration of the hydrophone and not to any meaningful
signal.
FIG. 3 conceptually illustrates one embodiment of the present
invention wherein the objectional effects of the liquid inertial
pressure are minimized.
The hydrophone of FIG. 3 includes a housing 30 having contained
therein a differential pressure sensor 32, such as a multi-laminar
bender disc, which separates the housing into two distinct chambers
33 and 34.
Acoustic waveguides 36 and 37 extend from the housing and include
respective pressure measuring ports 38 and 39 covered with
compliant members 40 and 41. The unit is filled with a transducer
fluid such as castor oil and the pressures at pressure measuring
ports 38 and 39 are communicated to respective sides of the
differential pressure sensor. As opposed to the arrangement of FIG.
2 however, the left acoustic waveguide 36 communicates with the
right chamber 34 by means of passageway 44, and the right acoustic
waveguide 37 communicates with the left chamber 33 by means of
passageway 45.
If the unit is now accelerated in the direction indicated, the
differential pressure sensor, due to its own mass, will deflect as
illustrated in FIG. 3A. Due to the novel arrangement, the liquid
pressure buildup due to the acceleration acts to deflect the
differential pressure sensor in the direction as indicated in FIG.
3B, a direction opposite to that deflection of FIG. 3A. In the
present arrangement, the differential pressure measuring device is
given a certain mass such that the inertial response of the liquid
on the sensor is approximately equal, and opposite, to the inertial
response of the sensor due to its mass. When this condition is met,
substantially no output signal will be provided by the sensor as a
result of acceleration.
FIGS. 4A through 4F depict liquid filled containers to illustrate
the principle of fluid pressure. The vertical container of FIG. 4A
contains a liquid of height H meters. If the ambient pressure is
P.sub.O, the pressure P.sub.H at the bottom of the container is
where:
P.sub.H and P.sub.O are measured in newtons/meter.sup.2
(Pascals);
g is gravitational acceleration in meters/sec.sup.2 ;
.rho. is the density of the liquid in Kg/meter.sup.3.
FIG. 4B illustrates a similar liquid filled container tipped on its
side and covered, on its right side by a compliant member. If the
container is accelerated with an acceleration a, in the direction
as illustrated, the pressure P.sub.X at the left end of the
container will be
where:
P.sub.O is the ambient pressure acting on the compliant member in
Pascals;
X is the length of the fluid column in meters.
FIG. 4C illustrates the container tipped to the left having a water
column of length Y. If the container is accelerated in the same
direction as was the case in FIG. 4B, the pressure P.sub.Y at the
right end of the container will be
It is to be noted that the pressure measurement is not a function
of the shape of the container. For example, for the serpentine
container illustrated in FIG. 4D, the pressure P.sub.X will be
identical to that of FIG. 4B and is defined by Equation (2).
If the two containers of FIGS. 4B and 4C are placed end to end as
in FIG. 4E, and the unit accelerated in the direction indicated,
the resultant differential pressure P at the interface due to the
liquid inertia will be
which reduces to
If, in FIG. 4E, the junction 50 were replaced by a bender disc, the
arrangement would be analogous to the prior art illustrated in FIG.
2. If the containers were curved as illustrated in FIG. 4F and the
junction 52 between them replaced by a bender disc, the arrangement
would be analogous to that illustrated in FIG. 3.
In the present invention, the total liquid inertial force acting on
the sensor is made equal to the inertial force of the sensor
assembly. That is, the liquid pressure times the area over which it
acts is the force equal to the mass of the disc assembly times its
acceleration. If a is the acceleration in meters/sec.sup.2, M the
mass of the assembly in Kg, and A the area in meters.sup.2 over
which the fluid is effective:
If a bender disc is used and its diameter is d, its effective
diameter will be 2/3d, such that its area will be
Substituting into Equation (6) and cancelling the acceleration
terms, the mass of the disc assembly required to counteract the
liquid inertia will be approximately
In all probability the sensor assembly will not have this exact
mass so that individual pieces of mass will be added to obtain the
quantity derived in Equation (8). As a practical matter then, the
resulting unit may be given a predetermined acceleration and if any
output voltage is provided due to that acceleration, the value of X
and/or Y may be adjusted to trim the apparatus and to minimize any
output signal due to vibration.
FIG. 5 illustrates, in an exploded view, a bender disc sensing
means which may be utilized herein. The bender disc is a
multi-laminar unit including a central metallic disc 60 made for
example of aluminum and having a thickness in the order of 0.01
inch. Cemented to either side of disc 60 are piezoelectric discs 62
and 63 also of 0.01 inch thickness. Since the resulting unit in
general would not have enough mass to satisfy the equality of
Equation (8), additional mass is added in the form of brass weights
65 and 66 and the assembly is held together by means of nut and
bolt 67, 68 with the brass weight 65 and 66 being spaced from
piezoelectric discs 62 and 63 by means of standoffs 70 and 71.
FIG. 6 is a plan view, with a portion broken away, of a dipole
hydrophone constructed in accordance with the teachings herein. The
hydrophone includes a housing 74 containing a differential pressure
sensing unit 76 identical to that described in FIG. 5 and which
divides the interior of housing 74 into two distinct chambers 78
and 79.
First and second acoustic waveguides 82 and 83 extending along a
central axis C are coupled to housing 74 by means of coupler
portions 85 and 86. The ends of the acoustic waveguides 82 and 83
constitute pressure sensing ports which are covered by respective
compliant members 88 and 89 held in place by securing rings 90 and
91.
The distance from the pressure measuring port at the end of
waveguide 82 to the center of the sensing unit is designated Y and
the distance from the pressure measuring port at the end of
acoustic waveguide 83 to the center of the sensing unit is
designated X. In order to balance the hydrophone in accordance with
Equation (8), the apparatus is constructed and arranged so that
distance X or Y or both may be varied. This is accomplished by the
threaded engagement of each acoustic waveguide with respective
coupler portions 85 and 86. Since the hydrophone is liquid filled,
if one or both of the waveguides is screwed in to shorten a
distance, one of a plurality of machine screws 94 is removed to
allow for liquid overflow. Conversely, if one or both of the
waveguides is moved to increase a distance, then additional liquid
may be added. If X and Y are of equal lengths and
(X+Y)<.lambda./2, the resulting beam pattern will be a pure
dipole as illustrated in FIG. 1. If X and Y are of unequal lengths
or if (X+Y)>.lambda./2, other lobes begin to appear in the beam
pattern.
For the plan view illustrated, coupler portion 85 includes an
elongated horizontal chamber or opening 96 by means of which liquid
in acoustic waveguide 82 is communicative with chamber 79 via
passageways 98 and 99.
Although not illustrated in FIG. 6, an elongated vertical chamber
or opening in coupler portion 86 will communicate liquid in
acoustic waveguide 83 through similar passageways to chamber 78.
Liquid baffles 102 and 103 in conjunction with gaskets 106 and 107
ensure that the liquid in the left waveguide is communicative with
the right side of the sensor, and the liquid in the right waveguide
is communicative with the left side of the sensor as was explained
with respect to FIG. 3.
FIG. 7 illustrates an exploded view of the hydrophone of FIG. 6
with a horizontal cross-section taken through housing 74; and FIG.
8 illustrates the housing with a vertical cross-section. All of the
elements of FIG. 6 are identified in the exploded view of FIG. 7
which additionally illustrates the mentioned elongated vertical
chamber or opening designated 110 in coupler portion 86. Liquid in
acoustic waveguide 83 is then communicative with left chamber 78
via passageways 117 and 115, better illustrated in FIG. 8. As can
be seen in FIG. 7, gaskets 106 and 107 include elongated slits 118
and 120 which line up with the respective elongated horizontal
chamber 96 and elongated vertical chamber 110.
In the actual construction of the hydrophone, the edge of the
central metallic disc of the sensor unit 76 would be secured to the
rim portion 122 such as by epoxy. Electrical connection to the
sensor unit would then be made through waterproof electrical
connector 125 mounted on housing 74. Although the acoustic
waveguides are illustrated as being threadedly engaged with the
coupler portions 85 and 86 to vary the distance between an acoustic
port and the sensing unit, other means of varying this distance may
be provided such as by telescopic sections or by a threadedly
engaged end section of waveguide, by way of example.
If the hydrophone is vibrated longitudinally, that is in an axial
direction, there is a chance of acoustic pressure buildup at the
pressure measuring ports 88 and 89. In order to reduce this
pressure buildup, the acoustic waveguide may be fabricated in
accordance with the design illustrated in FIG. 9. The end portion
of an acoustic waveguide 128 is illustrated and includes measuring
ports 130 covered by a compliant member 132. The waveguide includes
an extension 134 beyond the pressure measuring ports 30, and which
extension minimizes, if not eliminates, the pressure buildup
problem.
FIG. 10 illustrates another embodiment of the invention wherein the
hydrophone depicted has associated therewith the well known
cardioid beam pattern as illustrated in FIG. 11. The hydrophone
includes a housing 140 which contains a differential pressure
sensing means as previously illustrated, and first and second
acoustic waveguides 142 and 143 extend from the housing to
respective pressure measuring ports 146 and 147. The axial distance
from measuring port 146 to the center of the sensing unit is Y and
the axial distance from measuring port 147 to the center of the
sensor unit is X. The acoustic path length, however, from measuring
port 146 to the sensing element is greater than X by virtue of the
U-shaped bend. Let it be assumed that .tau..sub.1 is the time it
takes a pressure wave to travel in waveguide 143 from port 147 to
the sensor and .tau..sub.2 the time for a pressure wave to travel
in waveguide 142 from port 146 to the sensor. If .tau..sub.3 is the
time it takes for a pressure wave to travel from port 147 to port
146 externally in the water (distance X+Y) then in general a
cardioid beam pattern will be provided if the waveguide liquid and
waveguide lengths are chosen such that .tau..sub.1 =.tau..sub.2
+.tau..sub.3. Thus, as a variation, by eliminating one waveguide as
in FIG. 10A, .tau..sub.2 is made substantially equal to zero and a
cardioid pattern will result, while still maintaining inertial
balancing.
Suppose by way of example in FIG. 10 that a pressure wave as
indicated by line 150 is traveling in an axial direction relative
to the hydrophone, from right to left as indicated by the arrow. X
is chosen to be equal to Y and the length of waveguide 143 is
chosen to be 3X (from port to sensor). The pressure wave must
travel 3X within waveguide 143 until it reaches one side of the
pressure differential sensor. After the pressure wave 150 passes
measuring port 147, it will travel a distance of 2X in the water
until it reaches measuring port 146 after which the pressure is
communicated to the other side of the sensor after a travel of X in
waveguide 142. It is seen therefore that the same pressure signal
arrives at both sides of the differential pressure sensor at the
same time due the chosen path lengths and therefore no output
signal will be provided. This is in conformance with the beam
pattern of FIG. 11 wherein the hydrophone is assumed positioned at
point p. A wave emanating from the opposite direction as indicated
by pressure wave 152 will cause a pressure differential at the
sensing unit and it will be a maximum. Waves emanating from various
other directions will cause an output signal as governed by the
beam pattern.
Although both acoustic waveguides 142 and 143 do not extend along
the same axial line, the hydrophone will still provide inertial
balancing as previously described. In determining the mass to be
added to the differential pressure sensing arrangement, the form of
Equation (8) may still be utilized with X=Y. Acoustic waveguide 143
is illustrated by way of example as having a single U-shaped bend.
A multiple bend arrangement is more practical to conserve space and
the cardioid pattern will be provided as long as the multiple bend
waveguide is of a path length which will ensure cancellation of a
pressure wave such as 150.
With the critical value between path lengths, there is a
possibility that a standing wave in an acoustic waveguide may be
generated and degrade the response of the hydrophone. Accordingly,
in order to prevent these standing waves, the acoustic waveguides
are terminated at their ports with an acoustic resistance which is
made equal to the characteristic resistance of the waveguide. This
is completely analogous to terminating a transmission line in its
characteristic impedance to prevent standing waves.
FIG. 12 illustrates one example of an acoustic resistance
terminating an acoustic waveguide, waveguide 143. The acoustic
resistance is formed by a capillary opening 160 of a length l and
of a diameter b. The characteristic impedance of the waveguide is
given by the relationship
where:
Z is the characteristic impedance in ohms;
.rho. is the density of the waveguide liquid in Kg/meter.sup.3
;
C is the speed of sound in the liquid in meters/sec; and
.sigma. is the cross-sectional area in meters.sup.2.
The acoustic resistance of the capillary 160 is given by the
relationship
where:
R is the acoustic resistance in ohms;
k is a constant;
.mu. is viscosity of the waveguide liquid in Pascal-seconds;
l is the length of the capillary in meters;
b is the diameter of the capillary in meters.
Thus, knowing the waveguide liquid characteristics and waveguide
area, the characteristic impedance may be determined in accordance
with Equation (9).
The capillary is then designed according to Equation (10) where the
value of R is made equal to the value of Z calculated from Equation
(9).
The differential pressure sensor has been described by way of
example as a multi-laminar bender disc. The sensor however, can be
any one of a variety of differential pressure sensors such as a
condenser microphone, a velocity sensor on a disc, a group of
sensors, or even cylinders, to name a few. FIG. 13, and FIG. 13A
which is a view along line A--A of FIG. 13, illustrate a group of
sensors. A metallic disc 162 having a central aperture includes a
plurality of piezoelectric discs 164 on one side thereof and a
similar plurality of piezoelectric discs 165 on the other side
thereof. Disc 162 in conjunction with container 167 forms a
compartment which is communicative with acoustic waveguide 169.
Another chamber 171 is communicative with the other acoustic
waveguide 173. The principle of operation is identical to that
already described in that an axial acceleration or axial component
of acceleration to the right will tend to cause a deflection of the
sensing unit to the right due to the liquid, whereas an axial
acceleration or axial component of acceleration to the left will
tend to cause a deflection of the sensing unit to the left. By
proper choice of added weight, inertial balancing may be
accomplished.
FIG. 14 illustrates an arrangement which utilizes as the active
element, a piezoelectric cylinder 178. By means of passageway 180,
the left acoustic waveguide 182 is communicative with the inside of
cylinder 178 whereas the right acoustic waveguide 184 is
communicative with the outside of the piezoelectric cylinder via
passageway 186. Piezoelectric cylinder is positioned between an end
cap 188 and an added mass 190 supported by means of a spider 192.
Compliant rings 194 and 195 between the cylinder and mass 190 and
end cap 188 allow for normal transducer action.
The operation of the embodiment of FIG. 14 is such that when
accelerated, the mass 190 generates an axial stress causing the
generation of a voltage which is in opposition to the voltage
generated by the circumferential stressing due to the liquid
inertia force.
The voltage E.sub.O produced as a result of the liquid inertia
is
where:
P is the pressure difference across the cylinder wall in
Pascals;
d.sub.m is the mean diameter of the cylinder in meters;
g.sub.31 is the piezoelectric constant for the radially poled
cylinder in (volts-meters/newton).times.10.sup.-3.
Substituting the value of P from Equation (5)
where:
X and Y are the respective linear distances from the right and left
acoustic pressure measuring ports to the center of the
cylinder.
The voltage E.sub.m generated from the acceleration a of the sensor
unit is given by the relationship
where:
M is the value of mass in Kg of the cylinder 178 and mass 190.
The value of E.sub.O is equated to E.sub.m so that the total output
voltage due to the acceleration is zero. From Equations (12) and
(13)
Cancelling the "a" terms and solving for mass M
Equation (15) therefore gives the value, to a good approximation,
of the total mass needed for complete inertial balancing and
knowing the mass of the cylinder 178, the required added mass may
then be determined. As was the case with respect to the embodiment
previously described, the unit may be given a predetermined
acceleration and the distance X or the distance Y be adjusted so
that the total output voltage due to such acceleration is
substantially zero.
FIG. 15 illustrates another embodiment utilizing a piezoelectric
cylinder 196 and wherein the left acoustic waveguide 197 is
communicative with the outside of the cylinder and the right
acoustic waveguide 198 is communicative with the inside of the
cylinder by way of fluid ports 200. The cylinder is connected to an
end cap 102 by way of compliant ring 103 and is connected to a mass
106 by way of compliant ring 107. A diaphragm 109 isolates the
inside of the cylinder from fluid communication with the
outside.
It is recognized that for certain deployments the kydrophones may
experience other than linear acceleration. Thus where angular
acceleration will be encountered the hydrophones should be designed
to be symmetrical about a longitudinal axis as for example would be
the construction of the embodiments of FIGS. 6, 13, 14 and 15 (but
not that of FIG. 10).
For optimum inertial balancing in the linear acceleration case, as
is done in all embodiments described herein, there should be no
section of waveguide which would cause a differential output signal
due to uncompensated liquid pressure on the sensor. For example a
single right angle turn in one waveguide but not the other would
not necessarily cause an output when the hydrophone is accelerated
longitudinally, but would cause an output of acceleration were
along a direction perpendicular to the longitudinal axis.
* * * * *